A useful magic rule changes what a character can choose. A useful cost changes what they are willing to choose.
The difference matters. A power may look limited because it requires effort, a rare object, or a special person. If those limits never close an option or endanger something the character values, they are atmosphere rather than cost.
Build the system around six questions: What is the channel? Who has access? What is the limit? What is the cost? How can use fail? What social consequences follow? Then test the rule in a scene before adding another ability.
Decide what role magic plays
Magic can solve problems, create problems, reveal character, change social power, or make a theme tangible. It does not have to perform all those roles.
Patricia C. Wrede’s worldbuilding questions about magic and magicians ask about source, access, limits, cost, training, and social effects. Those questions are valuable because they push the writer beyond a list of spectacular outcomes.
Brandon Sanderson describes his own “laws” as guidelines rather than universal laws. His official explanation emphasizes that limitations can be more interesting than powers and recommends expanding what an existing ability can do before continually adding new ones. See Sanderson’s Laws of Magic.
Hard and soft magic are not moral categories. A system can remain mysterious to the reader while still having consistent consequences for the author. SFWA’s discussion of making soft magic systems work recommends deciding what role the magic plays rather than assuming every setting needs the same degree of explanation.
1. Define the channel
The channel is the material or action through which the impossible effect occurs. A narrow channel prevents magic from becoming a general answer.
In the Two-Dated Archive, the channel is fresh writing in the margins of paired surviving sheets. The channel is not any book, any old writing, or thought at a distance.
Ask:
- Does the channel have a physical location?
- Can it be guarded, misplaced, damaged, or misunderstood?
- Does using it leave evidence?
- Can ordinary people interact with the channel without controlling the effect?
The archive’s folios belong to the record world even when no message is moving through them. That gives the object mundane value and makes its custody consequential.
2. Define access
Access is not the same as magical talent. A person may understand the rule but lack the keys, permission, literacy, privacy, materials, or institutional standing required to use it.
For the archive, a character needs physical access to a paired surviving sheet, room in its margin, and the ability to write there. Institutions and relationships can therefore control use without possessing a separate supernatural ability.
This creates several positions:
- a skilled worker permitted to handle the object but not alter it;
- a custodian with authority but incomplete understanding;
- a correspondent who can answer but cannot control conditions in the other date;
- a person affected by a message who never sees the folio.
Access turns worldbuilding into conflict. It also keeps expertise, permission, and power from collapsing into one trait.
3. State the limit in testable language
“The magic is unpredictable” is difficult to plan around. A testable limit tells the writer what cannot happen.
The locked public rules of the Two-Dated Archive are:
- Only fresh marginal ink crosses.
- The interval is fixed at 103 years.
- The exchange requires paired surviving sheets.
- Drawings do not cross.
- Voices do not cross.
- People and objects do not cross.
- Margin space is finite.
- Erasure damages nearby ordinary writing in both dates.
These are inventions for this story world, not historical or occult claims. They should remain stable until a formal canon revision is approved.
The negative rules are as important as the positive one. They stop the archive from becoming a portal, an illustrated transmission system, a voice channel, or an object-delivery device whenever the plot becomes difficult.
4. Make the cost attack a value
A cost produces story when it threatens something the character is trying to protect.
Finite margin space makes every line compete with a possible later line. The cost is not merely “there is not much room.” It creates choices about whose question deserves space, whether a warning is worth the ambiguity of brevity, and when silence preserves future options.
Erasure makes correction dangerous. Removing transmitted ink damages nearby ordinary writing in both dates. If the folio contains evidence someone values, revising a message may destroy part of the very record the characters hope to protect.
These costs attack Livia’s values directly. She cares about the integrity of records, yet using the archive can consume or damage the record. That contradiction is more productive than a price paid in an unrelated resource.
5. Define failure without inventing a new effect
A failure mode answers: how can the known rule produce a bad result?
For this mechanism, failure can arise from ordinary causes and established limits:
- a message is too compressed to interpret safely;
- limited space is spent on a poor question;
- a sheet is unavailable, guarded, or no longer paired by survival;
- a reply arrives after someone has acted on incomplete information;
- erasure damages ordinary writing that mattered;
- access to the folio exposes the writer to accusation or coercion.
None of those failures adds a power. The archive does not suddenly translate languages, reveal thoughts, choose recipients, create visions, or send objects. The pressure comes from human interpretation, material custody, and the rules already declared.
6. Trace the social consequence
If a power can be used repeatedly, people will form expectations, restrictions, and strategies around it—once they know or suspect it exists.
Ask:
- Who would try to control the paired sheets?
- Who would decide which messages deserve scarce space?
- Would a transmitted line count as evidence, forgery, contamination, or blasphemy to different observers?
- Who bears responsibility for damage to the ordinary record?
- Can secrecy protect the users while harming people excluded from the exchange?
These are story questions, not established claims about a historical institution. They show how an invented rule can interact with custody, authority, and trust without rewriting real history as evidence for magic.
Stress-test the rule with abuse cases
An abuse case asks how a determined person might exploit the published rule. The purpose is to find pressure points, not hidden bonus powers.
Hoard the channel
A custodian restricts access to the paired sheet. The conflict concerns possession and permission; it does not require a magical countermeasure.
Consume the margin
A correspondent fills space with messages of low value or deliberate ambiguity. The known scarcity becomes a form of control.
Threaten the ordinary record
Someone writes where any later erasure would damage nearby evidence. The established erasure cost turns placement into leverage.
Exploit the interval
A person acts knowing a reply cannot escape the fixed 103-year relationship. The interval does not change; asymmetry of context creates the advantage.
After each test, ask whether the system still creates at least two costly choices. If one tactic makes every later problem trivial, narrow or clarify an existing rule before adding another.
Run the scene test
For any scene involving magic, complete five sentences:
- The character wants ______.
- The established rule permits __ but forbids ____.
- Access depends on ______.
- Using the rule risks __, which the character values because ____.
- The choice changes the next scene by ______.
A successful magic scene should reveal something about the chooser. SFWA’s essay on story as worldbuilding argues that invented worlds become meaningful when they force characters to grapple with themselves; that is a useful guard against rules delivered only as exposition.
Keep the system connected
See the World Building library for the broader setting catalogue, the Two-Dated Archive for the mechanism’s public record, and The Register That Writes Back for a spoiler-safe account of the pressure it creates. Livia Serrani shows why the cost is personal, while The Chronicles of Renaissance Scribes places it in the wider story.
The best test of a cost is not whether it sounds severe. It is whether a character can avoid paying it—and must give up something meaningful to do so.